From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Miller Subject: Re: GRO/GSO hiding PMTU? Date: Thu, 10 Feb 2011 15:07:59 -0800 (PST) Message-ID: <20110210.150759.59671055.davem@davemloft.net> References: <20110210.145555.39165146.davem@davemloft.net> Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: netdev@vger.kernel.org, netfilter-devel@vger.kernel.org To: herbert@gondor.apana.org.au Return-path: Received: from 74-93-104-97-Washington.hfc.comcastbusiness.net ([74.93.104.97]:47242 "EHLO sunset.davemloft.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752828Ab1BJXHX (ORCPT ); Thu, 10 Feb 2011 18:07:23 -0500 In-Reply-To: <20110210.145555.39165146.davem@davemloft.net> Sender: netfilter-devel-owner@vger.kernel.org List-ID: From: David Miller Date: Thu, 10 Feb 2011 14:55:55 -0800 (PST) > I suspect that the packet arrives on eth1, accumulates into GRO, and > thus marked as GSO as well, then GSO/TSO on output to eth0 is > re-segmenting things transparently, and we're not getting the ICMP > frag-needed message and the packet drop because of the skb_is_gso() > check in ip_forward(). > > if (unlikely(skb->len > dst_mtu(&rt->dst) && !skb_is_gso(skb) && > (ip_hdr(skb)->frag_off & htons(IP_DF))) && !skb->local_df) { > IP_INC_STATS(dev_net(rt->dst.dev), IPSTATS_MIB_FRAGFAILS); > icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED, > htonl(dst_mtu(&rt->dst))); > goto drop; > } > > So if that's what is happening, that's cute, but I think we need to > fix this :-) > > Perhaps the check in ip_forward() should instead validate the gso_size > in the skb_is_gso() case? > > That'd be a little tricky since gso_size is an MSS value whereas what > we're checking against (skb->len) is the full packet size, headers and > all. Nevermind, I turned off gso/tso on eth0 (outgoing interface) and it still happens. I guess netfilter or something else is causing this.